In a nuclear reactor such as a boiling water reactor (BWR), a reactor core containing nuclear fuel rods is provided for heating water to be used as a power source for a steam turbine-generator, for example. The fuel rods are typically grouped together in fuel bundles or assemblies, having a square matrix for example, with upper and lower tie plates being used to maintain a predetermined lateral spacing between the adjacent fuel rods.
In the BWR, a recirculating coolant, or water, is suitably channeled through the lower tie plate and upwardly between the fuel rods for cooling the fuel rods during operation, with the coolant having an increasing steam void fraction as it rises upwardly along the fuel rods with the resulting liberated steam being suitably channeled to the steam turbine.
In order to control reactivity of the fuel rods, conventional solid control rods are selectively translatable upwardly and downwardly between the fuel rods for selectively absorbing neutrons emitted therefrom during operation. The control rods may be in the conventional form of a cruciform disposed between adjacent fuel bundles, or may be in the form of a plurality of finger-type rods insertable in the fuel bundles between selected fuel rods thereof. In both examples, suitable control rod drives (CRDs) are also provided which may be located below the lower head of the pressure vessel or above the upper head of the pressure vessel depending upon the particular design. In both designs, however, the CRDs are disposed outside the pressure vessel and require suitable penetrations of the pressure vessel for the translatable plungers thereof to translate the control rods.
Concepts for controlling reactivity of the reactor core using a liquid neutron absorber in hollow tubes are known in the literature, with the level of the liquid absorber being selectively varied. This is analogous to the degree of insertion of the solid control rods into the reactor core. However, a liquid neutron absorber reactivity control system has practical problems associated with the installation of the required many tubes and reservoirs for the liquid absorber in the pressure vessel. Furthermore, the ability to replace components of such a system is also required which imposes even further practical problems for dealing with the substantial number of tubes and connections therebetween which require individual replacement with suitable leak tight connections. The liquid absorber must also be suitably separated from the circulating coolant in the reactor core to prevent the adulteration thereof which would adversely affect operation of the reactor core.